In carburettors which operate under a constant reduced pressure, it is known to provide a chamber including a main flap, on which an accelerator pedal acts, and a secondary flap which is situated in the chamber upstream of the main flap and which is biassed towards its closed position by a force which is proportional to the flow rate of air through the chamber, such that a reduced pressure is produced between the two flaps which remains practically constant. The secondary flap controls a needle-valve which controls the flow rate of fuel into the chamber, the fuel entering the chamber between the two flaps so as to control the fuel/air mixture produced in the chamber since the degree of opening of the secondary flap is a function of the air flow rate, the metering needle of said valve, which is connected mechanically to the said flap, also occupies a position defined by the rate of air flow into the carburettor.
However, the required fuel/air mixture varies in dependence upon the speed and the load of the engine and consequently one and the same air flow rate can correspond to very different running conditions. For example, the air flow rate will be the same at 1,500 revolutions/minute and full load as at 3,000 revolutions/minute and half load or 6,000 revolutions/minute and quarter load. Thus, the most suitable rate of metering fuel varies with these different running conditions.
If the profile of the needle in the needle valve is arranged to provide metering (depending on the shift of the main flap under the effect of the air flow rate), which results in a very low carbon monoxide CO content in exhaust gasses from the engine for all partial loads of the engine and for all speeds less than 4,000 revolutions/minute, the engine runs rather unsatisfactorily under conditions of acceleration and under full load conditions due to a weak mixture.